The present invention relates to a transmitter. More specifically, the present invention relates to transmitter using a phase modulating and combining circuit.
The variety of portable electronic devices as well as the usage of such devices has dramatically increased in recent years. This is especially true of mobile handsets, which are presently used by a large proportion of the populations in the United States, Europe, and Japan, for example. Mobile handsets contain modulators that enable wireless transmissions by permitting complex modulation formats that support modern, high-data-rate wireless communications.
As in a vast majority of portable electronic devices, there is a demand to decrease the cost, size and number of parts in mobile handsets and the components used therein. However like other components, the transmitters used in mobile handsets have multiple types of problems that must be addressed in the design stage. Inevitably, these design solutions limit the amount of cost, part and size reduction that can take place.
One example of particular problems faced by transmitters is precise control of transmitter power. Control of this power is desirable at least to avoid power wastage and increase the amount of usable time before the battery needs to be recharged as well as decreasing the amount of interference. As most of the circuitry in transmitters is integrated, to improve semiconductor yield and lower the manufacturing cost, meeting the power specifications between the transmitter and cellular tower for all transmitter process variations during manufacture is relatively difficult. In addition to taking account of the effects of process variations on output power, the transmitter power specification also takes into account large changes in the ambient temperature when using the mobile handset in order to meet the communication standard used by the mobile handset. This is because as the ambient temperature varies, the transmitting power of the mobile handset varies concomitantly; for example, if the transmitting power is too high, the signal can bury other signals used in communications with the cell tower.
Besides precise power control, other problems of the transmitter are addressed in designing the transmitter architecture. These additional problems include relatively low third-order intermodulation and high phase noise in signals from the transmitter. Third-order intermodulation becomes more problematic as the signal is usually amplified between the modulator and the antenna, causing interference between adjacent channels. Accordingly, having a high third-order intermodulation improves the adjacent channel power rejection.
In addition to third-order intermodulation requirements, low noise power output at some specified frequency offset from the carrier is required for most communication protocols, such as the Global System for Mobile Communications (GSM) or Code Division Multiple Access (CDMA) to reduce interference in the receiver. Also, in conventional transmitter modulators, a harmonic-rich square wave is applied to the modulator as a local oscillator signal. This results in an output that is also rich in harmonics and requires filtering before being transmitted by the antenna.
A surface acoustic wave (SAW) filter disposed between the modulator and power amplifier is used to decrease the phase noise and harmonics of the signals from the modulator to acceptable levels in conventional transmitters. SAW filters have an inherent insertion loss associated with introducing the filter into the signal stream. This insertion loss is relatively large, usually about 3 dB. Use of a SAW filter also increases the size and the cost of the overall transmitter. Further, the SAW filter has inherent power limitations which may become problematic depending on the transmitter design; if the signal supplied to the SAW filter is too large, the filter can be damaged and, accordingly, then fail to operate properly.